MEXICO CITY – Scientists say the amount of Monarch butterflies wintering in Mexico dropped 59 percent this year, falling to the lowest level since comparable record-keeping began 20 years ago.

It was the third straight year of declines for the orange-and-black butterflies that migrate from the United States and Canada to spend the winter in mountaintop fir forests in central Mexico. Six of the last seven years have shown drops, and there are now only one-fifteenth as many butterflies as there were in 1997.

The millions of Monarchs cluster so densely on tree boughs that researchers don't count their individual numbers but rather measure the amount of forest they cover. This winter, they covered just 2.93 acres (1.19 hectares), down from 7.14 acres (2.89 hectares) last year.